Keywords: 
• 逆韧致辐射 /
• 无辐射跃迁 /
• 雪崩电离 /
• 非线性折射率

Abstract: The processing morphology of cubic crystal LiF irradiated by femtosecond laser varies with the polarization direc-tion. When the polarization direction is parallel to the crystal orientation?110?, the distance between the starting point and the surface is 1.08 times that along ?100? polarization, and the distance between the end point and the surface is 1.01 times. While the cubic crystal is irradiated by a femtosecond laser, self-focusing and inverse bremsstrahlung are two probable mechanisms dependent on polarization. In order to investigate the relation between the self-focusing and polarization, in this paper we report the nonlinear refractive index n2 of LiF crystal which is linear with respect to self-focusing coe?cient. The Z-scan technique is used to measure the nonlinear refractive indexes at different polarizations. As the polarization direction is rotated from?110?to?100?, the nonlinear refractive index decreases, and the self-focusing effect becomes weaker. If self-focusing leads to the dependence of morphology on polarization, the distance between the starting point and the surface for ?100? polarization should be longer than that for ?110? polarization. However, the experiment exhibits an opposite result that the distance between starting point and the surface for?100?polarization is shorter than that for?110?polarization. Therefore, the processing morphology which changes with polarization is not a consequence of the self-focusing. So in order to understand why the processing morphology varies with polarization, in this paper we present a model which combines inverse bremsstrahlung, avalanche ionization and radiationless transition. We believe that the recombination due to radiationless transition has a great effect on laser machining. The inverse bremsstrahlung coe?cient of ?110? polarization is less than that of ?100? polarization, as a result, the density of free electrons which are produced by inverse bremsstrahlung and avalanche ionization at?110?polarization is less than that at?100?polarization. At first, the laser energy is transferred to the free electrons by inverse bremsstrahlung and avalanche ionization, which is described by the paraxial nonlinear Schrodinger equation and evolution equation of electron density. The density of free electrons is obtained by solving the equations. Then free electrons transfer the energy to the crystal lattice in the process of recombination through radiationless transition, which is depicted by energy conservation and gives the distribution of lattice temperature along the propagation direction. Finally, the area in LiF crystal of which the lattice temperature climbs up to above the melting point is processed. According to the simulation, the distance between the starting point and the surface at?110?polarization is 1.03 times that at?100?polarization, and the distance between the end point and the surface at?110?polarization is 0.981 times that at?100?polarization. These are consistent with the experimental results. Simulation and experimental results demonstrate that the inverse bremsstrahlung, which is dependent on polarization, is the main reason for morphology changing with the polarization of femtosecond laser. These research results may contribute to inducing microstructure in transparent dielectrics through femtosecond laser.